There are a wide variety of molding systems to produce parts of thermoplastic or thermoset resins, or thermoplastic or thermoset composites. In vacuum molding, a slab (constant thickness sheet) of heated polymeric material is placed on the vacuum mold and a vacuum drawn between the mold and the heated plastic material to draw the plastic material onto the mold. Similarly, in compression molding, a lump or slab of preheated material is pressed between two molding forms that compress the material into a desired part or shape.
Compression Molding
Compression molding is by far the most widespread method currently used for commercially manufacturing structural thermoplastic composite components. Typically, compression molding utilizes a glass mat thermoplastic (GMT) composite comprising polypropylene or a similar matrix that is blended with continuous or chopped, randomly oriented glass fibers. GMT is produced by third-party material compounders, and sold as standard or custom size flat blanks to be molded. Using this pre-impregnated composite (or pre-preg as it is more commonly called when using its thermoset equivalent), pieces of GMT are heated in an oven, and then laid on a molding tool. The two matched halves of the molding tool are closed under great pressure, forcing the resin and fibers to fill the entire mold cavity. Once the part is cooled, it is removed from the mold with the assistance of an ejecting mechanism.
Generally, the matched molding tools used for GMT forming are machined from high strength steel to endure the continuous application of the high molding pressure without degradation. These molds are often actively heated and cooled to accelerate cycle times and improve the surface finish quality. GMT molding is considered one of the most productive composite manufacturing processes with cycle times ranging between 30 and 90 seconds. Compression molding does require a high capital investment, however, to purchase high capacity presses (2000-3000 tons of pressure) and high-pressure molds, therefore it is only efficient for large production volumes. Lower volumes of smaller parts can be manufactured using aluminum molds on existing presses to save some cost. Other disadvantages of the process are low fiber fractions (20% to 30%) due to viscosity problems, and the ability to only obtain intermediate quality surface finishes.
Injection Molding
Injection molding is the most prevalent method of manufacturing for non-reinforced polymeric parts, and is becoming more commonly used for short-fiber reinforced thermoplastic composites. Using this method, thermoplastic pellets are impregnated with short fibers and extruded into a closed two-part hardened steel tool at injection pressures usually ranging from 15,000 to 30,000 psi. Molds are heated to achieve high flow and then cooled instantly to minimize distortion. Using fluid dynamic analysis, molds can be designed which yield fibers with specific orientations in various locations, but generically injection molded parts are isotropic. The fibers in the final parts typically are no more than one-eighth (⅛)″ long, and the maximum fiber volume content is about 40%. A slight variation of this method is known as resin transfer molding (RTM). RTM manufacturing utilizes matted fibers that are placed in a mold which is then charged with resin under high pressure. This method has the advantages of being able to manually orient fibers and use longer fiber lengths.
Injection molding is the fastest of the thermoplastic processes, and thus is generally used for large volume applications such as automotive and consumer goods. The cycle times range between 20 and 60 seconds. Injection molding also produces highly repeatable near-net shaped parts. The ability to mold around inserts, holes and core material is another advantage. Finally, injection molding and RTM generally offer the best surface finish of any process.
The process discussed above suffers from real limitations with respect to the size and weight of parts that can be produced by injection molding, because of the size of the required molds and capacity of injection molding machines. Therefore, this method has been reserved for small to medium size production parts. Most problematic from a structural reinforcing point is the limitation regarding the length of reinforcement fiber that can be used in the injection molding process.
Composites and other Processes
Composites are materials formed from a mixture of two or more components that produce a material with properties or characteristics that are superior to those of the individual materials. Most composites comprise two parts, a matrix component and reinforcement component(s). Matrix components are the materials that bind the composite together and they are usually less stiff than the reinforcement components. These materials are shaped under pressure at elevated temperatures. The matrix encapsulates the reinforcements in place and distributes the load among the reinforcements. Since reinforcements are usually stiffer than the matrix material, they are the primary load-carrying component within the composite. Reinforcements may come in many different forms ranging from fibers, to fabrics, to particles or rods imbedded into the matrix that form the composite.
There are many different types of composites, including plastic composites. Each plastic resin has its own unique properties, which when combined with different reinforcements create composites with different mechanical and physical properties. Plastic composites are classified within two primary categories: thermoset and thermoplastic composites.
Thermoset composites use thermoset resins as the matrix material. After application of heat and pressure, thermoset resins undergo a chemical change, which cross-links the molecular structure of the material. Once cured, a thermoset part cannot be remolded. Thermoset plastics resist higher temperatures and provide greater dimensional stability than most thermoplastics because of the tightly cross-linked structure found in thermoset plastic. Thermoplastic matrix components are not as constrained as thermoset materials and can be recycled and reshaped to create a new part.
Common matrix components for thermoplastic composites include polypropylene (PP), polyethylene (PE), polyetheretherketone (PEEK), polyether imide (PEI), polyethylene terephthalate (PET), acrylonitrile-butadiene-styrene (ABS) and polyamide (nylon). Thermoplastics that are reinforced with high-strength, high-modulus fibers to form thermoplastic composites provide dramatic increases in strength and stiffness, as well as toughness and dimensional stability.
Molding Methods for Thermoplastic Composites Requiring “Long” fibers
None of the processes described above are capable of producing a thermoplastic composite reinforced with long fibers (i.e., greater 12 millimeters) that remain largely unbroken during the molding process itself; this is especially true for the production of large and more complex parts.
Historically, a three-step process was utilized to mold such a part: (1) third party compounding of pre-preg composite formulation; (2) preheating of pre-preg material in oven, and, (3) insertion of molten material in a mold to form a desired part. This process has several disadvantages that limit the industry's versatility for producing more complex, large parts with sufficient structural reinforcement.
One disadvantage is that the sheet-molding process cannot produce a part of varying thickness, or parts requiring “deep draw” of thermoplastic composite material. The thicker the extruded sheet, the more difficult it is to re-melt the sheet uniformly through its thickness to avoid problems associated with the structural formation of the final part. For example, a pallet having feet extruding perpendicularly from the top surface is a deep draw portion of the pallet that cannot be molded using a thicker extruded sheet because the formation of the pallet feet requires a deep draw of material in the “vertical plane” and, as such, will not be uniform over the horizontal plane of the extruded sheet. Other disadvantages associated with the geometric restrictions of an extruded sheet having a uniform thickness are apparent and will be described in more detail below in conjunction with the description of the present invention.
A series of U.S. Pat. Nos. (the Polk patents) 7,208,219; 6,900,547; 6,869,558; and 6,719,551 describe molding systems for producing a thermoplastic resin of thermoplastic composite parts using either a vacuum or compression mold with parts being fed directly to the molds from an extrusion die while the thermoplastic slab still retains the heat used in heating the resins to a fluid state for forming the sheets of material through the extrusion die. These patents describe a thermoplastic molding process and apparatus using a thermoplastic extrusion die having adjustable gates (dynamic dies) for varying the thickness of the extruded material, which material is molded as it is passed from the extrusion die. In addition they describe a continual thermoforming system that is fed slabs of thermoplastic material directly from an extruder forming the slabs of material onto a mold that can be rotated between stations.
The thermoplastic material is extruded through an extrusion die that is adjustable for providing deviations from a constant thickness plastic slab to a variable thickness across the surface of the plastic slab. The variable thickness can be adjusted for any particular molding run or can be continuously varied as desired. This allows for continuous molding or thermoplastic material having different thickness across the extruded slab and through the molded part to control the interim part thickness of the molded part so that the molded part can have thick or thin spots as desired throughout the molded part.
The technology of the aforementioned patents has been extremely useful for the production of large parts and for the production of parts made up of composite materials. In particular, the use of these technologies has allowed a “near net shape” deposition of molten composite material into the lower half of mold sets. Since the filled half of the mold represents a “near net shape” of the final molded part, the final compression molding step with the other half of the matched mold can be accomplished at very low pressures (<2000 psi) and with minimal movement of the molten composite material.
As thermoplastic demands continue to grow there is a growing need to occasionally build even larger parts. The use of systems as described in U.S. Pat. Nos. 7,208,219, 6,900,547, 6,869,558 and 6,719,551, are extremely useful for producing fairly large parts via low pressure molding of complex geometries but moving beyond those sizes would require the use of extremely large and expensive presses for compression molding. There is a need for a new approach, which can produce these much larger parts without the need to use extremely large presses.
The development described herein can provide all of the flexibility and capability for producing large and complex geometries from long-fiber reinforced plastic materials and the use of either thermoplastic or thermoset polymers without the use of larger presses.